Seems we need to set host_arch and target_arch more forcefully

chromiumcontent.gypi

@@ -8,11 +8,6 @@
     # tcmalloc.
     'linux_use_tcmalloc': 0,
     'conditions': [
-      ['OS=="mac"', {
-        # Build 64-bit.
-        'host_arch': 'x64',
-        'target_arch': 'x64',
-      }],
       ['OS=="win"', {
         # Chrome turns this off for component builds, and we need to too. Leaving
         # it on would result in both the Debug and Release CRTs being included in

script/update

@@ -124,6 +124,9 @@ def run_gyp():
     os.environ['GYP_GENERATORS'] = 'ninja'
 
     if sys.platform == 'darwin':
+        # Build 64-bit
+        os.environ['GYP_DEFINES'] = ' '.join(['host_arch=x64 target_arch=x64', os.environ.get('GYP_DEFINES', '')])
+
         # Use Xcode's own clang instead of Chromium's. This matches what embedding
         # applications will use, reducing the risk of bugs due to version
         # mismatches.